1. Field of the Invention
This invention relates to a water tank purifier for circulating and filtering water in a water tank for goldfishes, tropical fishes, etc. to clean up filth, such as the feces of fishes, the remainder of their food and various germs. This invention relates also to a water tank filter used in a water tank purifier.
2. Description of the Related Art
Various types of water tank purifiers have hitherto been manufactured for removing filth from tank water by circulating it continuously through a filter casing containing a filter. One type of purifier has its filter casing installed in a water tank. This type of purifier relies upon a rising water stream produced by bubbles rising from a bubble generator in the tank, or upon a motor pump for causing water in the tank to flow into the filter casing. Another type of purifier has its filter casing installed outside (for example, above) the water tank. This type of purifier relies upon a water drawing pipe having a motor pump for drawing up tank water into the filter casing.
Japanese Patent Office Official Gazette JP-A-2003-96 describes a water tank filtering apparatus having a filter casing installed in a water tank and relying upon a rising water stream produced by bubbles rising from a separately installed bubble generator to cause water in the tank to flow into the filter casing.
Referring in further detail to the water tank filtering apparatus mentioned above, it has a water suction and guide passage (19) opening in the lower portion of the filter casing and an upper water suction hole (16) opening in the upper portion of the filter casing as suction ports for introducing tank water into the filter casing. Coarse pieces of filth settling at or near the bottom of the water tank, such as feces and remaining food, are drawn in through the water suction and guide passage (19). Fine pieces of filth not settling, but floating in the tank water, such as various germs and small fragments of filth (small fragments of feces and food), are drawn in through the upper water suction hole (16). The filter casing contains two layers of filter material, i.e. an upper layer (4a) and a lower layer (4b) of filter material, the upper layer (4a) being packed with such material as fine particles of activated carbon, while the lower layer (4b) is packed with such material as coarse particles of gravel. A rising water stream is produced by bubbles rising in an extension pipe (11) situated in the center of the filter casing, so that tank water is drawn in through the upper water suction hole (16) and the water suction and guide passage (19), is purified by passing through the upper and lower layers (4a) and (4b) of filter material successively, and is discharged through a pipe hole (11a).
The filth occurring in a water tank is generally a mixture of readily settling coarse pieces, such as the feces of fishes and remaining food, and readily floating fine pieces, such as various germs and small fragments of filth. It is necessary to remove both coarse and fine pieces of filth thoroughly in order to ensure both the beauty of a water tank and the health of the fishes kept therein.
The filter used in a common water tank purifier has a single pore size, as it uses only one kind of filter material. Therefore, it has been unsatisfactory in that (a) it allows fine particles of filth to pass through if the filter material has a large pore size, while (b) it is clogged with coarse pieces of filth in a short time and has to be changed frequently if the filter material has a small pore size.
The water tank filtering apparatus described in Official Gazette JP-A-2003-96 has two layers of filter material, i.e. a lower layer (4a) of filter material for removing coarse pieces of filth and an upper layer (4b) of filter material for removing fine particles of filth in order to overcome the inconveniences as pointed out at (a) and (b) above. Thus, it claims to be capable of removing both coarse and fine particles of filth satisfactorily. It has, however, a serious problem as pointed out below.
When filth is introduced into the filter casing, it is divided into easily settling coarse pieces of filth drawn through the water suction and guide passage (19) at the bottom of the water tank, and easily floating fine particles drawn through the upper water suction hole (16) in the upper portion of the water tank. Both coarse and fine particles of filth in the tank water introduced into the filter casing, however, reach the upper and lower layers (4a) and (4b) of filter material. In other words, filth is not divided into coarse pieces guided to the lower layer (4b) of filter material having a large pore size and fine particles guided to the upper layer (4a) having a small pore size. Furthermore, both coarse and fine particles of filth first reach the upper layer (4a) of filter material having a small pore size. As a result, the upper layer (4a) is clogged with coarse pieces of filth in a very short time and has to be changed or cleaned frequently.
Even if a change in design may be made to allow the tank water introduced into the filter casing to pass first through the lower layer (4b) of filter material and then through the upper layer (4a), both coarse and fine particles of filth reach all the layers of filter material. Therefore, such design change still does not make the filter as a whole less likely to be clogged with coarse pieces of filth.